Conventional two-dimensional (2-D) optical data storage is accomplished by using photonic excitation of the storage material in the visible or infrared wavelengths at the diffraction limit. The theoretical limit for storage density in a 2-D system is typically ˜108 bits/cm2. With the presently growing need for economically viable, high performance computers with increased memory and storage capacity, optical storage media that provide the capability writing and reading data in a three-dimensional (3-D) format (with a theoretical storage capacity of >1012 bits/cm3) offers a potential solution for enabling compact, low-cost, high-speed memory devices with high storage capacities. Optical storage of information inside the volume of a 3-D storage or “memory” device is usually accomplished by inducing chemical changes via photonic processes (typically in the ultraviolet range) in micro-domain areas in a 3-D mode within an optical storage material comprising an optical storage media. “Optical storage materials” as used herein refers to materials (including chemical compounds) that are capable of undergoing a photo-induced change that can be subsequently monitored. “Optical storage media” as used herein refer to suitable forms and configurations of the optical storage materials so as to render them capable of presenting themselves in a 3-D matrix to an optical source in a manner that enables irradiation of micro-domains within the matrix in a pre-determined pattern representing an information set. The information (or data) storage process is termed “writing”, which denotes irradiation of the photon-absorbing “write” form of the molecules within the storage medium, transforming them into either a visible-light-absorbing form, or cause a change in their light transmission properties. Molecules that have undergone these changes are denoted to be in the “written” form. The stored data is accessed (or retrieved) by “reading”, which refers to probing the “written” form of the molecules with a photon source and by monitoring their optical response such as emitted fluorescence or changes in transmission due to altered refractive index.
Presently, two approaches are adopted for providing 3-D optical storage memory in materials. The first involves optically writing in micro-domains within the optical storage material whereby a change in the local refractive index of the material is introduced in the written micro-domains. The differential refractive index pattern is then subsequently read using standard optical methods. The second approach is to use optical storage materials that either include photochromic molecules, or polymers that are capable of undergoing photobleaching that may be additionally doped with chromophores such as dye molecules. Optical changes are photonically introduced in the dye molecules causing them to emit radiation in the visible range that are subsequently monitored. Although such materials are theoretically capable of providing bit densities of terabits per cubic centimeter, they have not been proven to be commercially viable for providing stable 3-D storage media.
Conventional 3-D optical storage media may be categorized into two types. The first constitutes a recordable medium comprising a stack of 2-D bit arrays that multiply the data density by the number of planes in the resulting 3-D stack. This type of optical data storage and retrieval processes in storage media has been conventionally accomplished by utilizing a multi-photon excitation process. It involves a two-photon excitation step to initiate the “write” process, whereby a photochemical reaction is induced in micro-domains within the medium that induces a permanent chemical change in the said domains. For optical storage media that are comprised of a polymeric matrix impregnated with a photoactivatable dye, the chemical change involves transformation of the dye, for example, into a fluorescent moiety. The written micro-domains comprising stored data can be subsequently “read” by photo-irradiation that causes them to emit fluorescence. U.S. Pat. Nos. 4,466,080 and 4,471,470 disclose the use of a plurality of intersecting beams to localize the writing and reading of information in 3-D photochromic optical memory media. U.S. Pat. No. 5,034,613 discloses a more simplified method that utilizes a single highly focussed beam to record and read information via a two-photon excitation. A two-photon photoactivation of a fluorescent dye that is non-fluorescent until photochemically modified has been also reported. However, the useful lifetime of such two-photon excitation processes that rely on fluorescence modulation is limited by photobleaching that occurs with multiple reads of the written data. This limitation may be attributed to the fact that the photoexcitation energies for the read process in these media induce photochemical degradation or crosslinking of the polymeric matrix and render them optically less transmissive. Thus the data stored within the media can no longer be read efficiently by the optical source. Another major limitation in this type of media is “cross-talk” between the planes, wherein the excitation beam strongly contaminates the planes above and below the focal plane on which data is being written. Since writing with 3-D resolution in these stacked array systems is accomplished by a nonlinear two-photon excitation of the medium to confine data storage to the focal plane, such contamination or “cross-talk” seriously limits both storage capacity and integrity of stored data.
The second approach for providing 3-D optical storage media uses storage materials that are capable of undergoing a photochemically induced localized change in material refractive index during the “write” process. This is accomplished by initiating a photochemical reaction such as photo-crosslinking in micro-domains within the material, thereby causing a localized change in the material refractive index of said domains relative to the surrounding media. These changes are subsequently “read” by an optical source that is capable of recognizing the change in light transmission properties within the material caused by the alteration in refractive index. U.S. Pat. No. 5,289,407 discloses a technique for writing and reading data in a three dimensional multi-layered format wherein information is written as submicron voxels (i.e. domains that are processible by means of visualizing 3-D shapes and structures by utilizing a series of cross-sectional images) of modified refractive index domains that are induced by a photo-crosslinking reaction initiated by a two-photon excitation process of a polymeric medium. An array of optically refractive “beads” is formed in a plurality of stacked planes. The written information is subsequently read with 3-D resolution using differential interference contrast microscopy. Although such storage media are not susceptible to photobleaching as in the case of the fluorescent dye impregnated type, their limitations include the following: 1) The photopolymer comprising the medium has to be irradiated with UV light prior to its use for writing optical data to gel the sample in order to prevent distortion due to shrinkage and flow, and 2) This pre-gelation process typically results in crosslink densities within the polymeric medium that are non-uniform, thereby resulting in varied refractive indices within the bulk medium. This, in turn, can result in reduced storage capacity due to domains of high crosslink density or “dead-spots” within the gel that are non-writable, as well as substantial loss in sensitivity for reading stored data.
Another drawback in existing optical data storage and retrieval methods in conventional media is that the photon excitation energies for the write and the read process are substantially similar. In dye-doped polymer media, for example, the data writing process typically involves a two-photon excitation of the impregnated dye matrix in micro-domains so as to render the dye in the said domains to become it fluorescent. The written micro-domains are subsequently read by photoexcitation of the fluorescent domains and analyzing their emission patterns. Since the “read” process requires photon energies that are similar in magnitude as those required for writing, written data can be contaminated by “overwriting” during the read process. This limitation is inherent in optical storage media that function via a photo-induced optical refractive index change during the write process, since the subsequent optical read process can cause contamination or “overwrite” problems, resulting in data degradation (“memory loss”) over multiple read cycles.